Just to say, I've had this issue a few times in the past week.
I'm using Xcode 6.4 with OS X Yosemite (v10.10.2), and from time to time, it just hangs on the "Fetching list of teams" message.
Most of the time, it works fine, but then it'll start to time-out, or throw an error like this:
The solution: reboot my MacBook. It fixes the issue every time.
It's odd. All other applications can happily access the internet, but for just this one part of Xcode, it seems unable to fetch the data it needs, and only a reboot sorts it out. I didn't have this issue before.
And it's hard to identify if this is an issue with Xcode 6.4 or OS X Yosemite, and Xcode demanded that I upgrade to the latest OS before it'd let me update to 6.4 (so I could actually debug an issue with iOS 8.2) so both updates happened on the same 1-2 days.
Bug ? Bug ? Did I say "bug" ? Course not. It's an undocumented feature.
(Sigh.)
Five hours later
I've just had the same issue again...
However, this time, I turned off WiFi on my MacBook, then turned Wifi back on again.
And the problem went away.
It (almost instantly) managed to load the list of (two) teams that I belong to.
Hope this helps.